1. Field of the Invention
This invention relates to web transports and, more particularly, to a method and apparatus for controlling the speed of a web.
2. Description of Prior Art
In the production of discrete sheet material from a continuous web, it is often necessary to momentarily stop the advance of the web sufficiently long to effect a cut across its width, an operation often referred to as chopping. It is, however, undesirable, and usually impractical to interrupt the web motion all along the web length since a number of upstream operations, such as casting, coating, printing or simply unrolling, etc. occur on a continuous basis. Therefore, it is usual to provide for a web accumulation section, which in its simpler form may comprise just one dancer roll loop having a dancer roll between the chopper and the output from the last operation on the web which requires continuous feed. So long as the average amount of web withdrawn from the accumulator by the intermittent motion on the output side, equals the amount introduced in the accumulator by the continuous feed, the operation of the chopper will not interfere with the upstream motion of the web.
To obtain the synchronization necessary between the web sections, it is known to employ speed controls which often rely on the position of the dancer roll to detect any mismatch between the demand and supply of web to that section.
As the web movement in the chopper section is stopped for the cutting operation, the dancer roll descends to accommodate the incoming web from the upstream side by increasing the path length of the web within the web accumulator. When web movement resumes in the chopper section, the web is taken out of the web accumulator at a faster rate than the input feed and the dancer roll ascends. A position transducer connected to the dancer roll generates a cyclic electrical signal having a direct component representative of the average dancer roll position, on which is superimposed an alternating component known in the art as "ripple"; this latter component represents the instantaneous position of the dancer roll around its average position. The ripple is undesirable in speed control systems, as it tends to continuously adjust the feed speed to compensate for the instantaneous variations of the dancer roll position.
A mechanical system with large backlash between the transducer and the dancer roll is sometimes used to minimize the ripple transmitted to the transducer. In the alternative, heavy electrical filtering on the output of the transducer may be employed to reduce the alternating voltage component of the generated signal. Neither approach provides an ideal solution as both introduce considerable delay in the system response, which results in instability problems in the related feed control.
While not specifically designed for web transport systems, it is also known in the art to remove ripple from a direct current using inverse feedback as shown in U.S. Pat. No. 3,474,349. This method requires the use of an amplifier with an electronic feedback to invert and amplify the alternating component only, of the output signal which is then reintroduced in the main amplifier input to diminish the ripple. The disadvantage of this approach is the feedback is not actually related to actual web movement and can produce false control signals.
Accordingly, it is an object of this invention to obviate many of the disadvantages of the prior art web speed control systems. Another object of this invention is to provide a simple method and apparatus for reducing ripple in the output of a transducer indicating the position of a dancer roll in a web transport system.